Saturable reactor voltage control circuit



July 26, 196e D. w. BARGEN ET AL 3,263,158

SATURABLE REACTOR VOLTAGE CONTROL CIRCUIT Filed Aug. l5, 1965 Z C'. d 775 am /f/arf 3,263,158 SATURABLE REACTGR VOLTAGE CONTROL CIRCUIT David W. Bargen, Scottsdale, and Bradford 0. Van Ness, Phoenix, Ariz., assignors to Motorola, Inc., Chicago, lll., a corporation oi Illinois Filed Aug. 15, 1963, Ser. No. 302,409 7 Claims. (Cl. 323-44) The present invention relates generally to control electric systems for electrical equipment, and it relates more particularly to an improved protective system for use in conjunction with positive temperature coefficient thermistors for de-energizing an electric motor when'the temperature of the motor windings exceeds a predetermined maximum threshold.

It is usual in the prior art to provide control circu-its for electric motors, and the like, for protecting the motors from the effects of overheating. These prior art systems usually include thermal overload relays, each of which, in turn, usually includes a terminal responsive unit. 'Ilhe thermal responsive unit is a `lai-metallic element which is heated directly, or indirectly, by the electric current in the windings of the motor to be protected.

The prior art thermal protective systems described briefly in the previous paragraph are not entirely sat-isfactory, mainly because the thermal characteristics of the relay do not match the thermal characteristics of Ithe machine to be protected.

Because of the above noted inadequacies of the thermal type of rel-ays in motor protection systems, other equivalent types of thermal protective circuits have been developed in .the prior art. These latter circuits respond directly to the heat generated by the windings of the motor to be protected. The latter type of prior art motor protective system is preferable because it can be made to respond directly to the temperature of the windings of the protected motor, and to de-energize the motor windings when the temperature thereof exceeds a predetermined safe maximum threshold.

An improved type of the latter prior -art system has been made commercially practicable by the provision of the relatively inexpensive positive temperature coefficient thermistor. This thermistor exhibits a small change in its resistance in the positive sense for increases in temperatures in the normal temperature range usually encountered in electric motor windings when the motor is running under normal conditions. However, the positive temperature coeilicient thermistor referred to above has switchlike characteristics, and it exhibits a l-arge increase in resistance when the winding temperature of the motor increases to an abnormally high level indicative of la malfunction therein.

Thermistors of this type are described, for example, in British Patent 714,965, d-a-ted September 8, 1954, and in an article by H. A. Sauer .and S. S. Flaschen, Proceedings Components Symposium, May 1, 1956, pages 41-46.

The present invention provides a solid state control circuit which lhas particular utility in motor protective systems of the type described above for operation in conjunction with positive temperature coeiiicient .thermistors For that reason, the control circuit of the invention will be described in such an environment. It will become evident as the description proceeds, however, that the control circuit of the invention -has general utility in the control or protection of electrical equipment, land that it can be made to respond to a wide variety of sensors, other than temperature responsive elements..

It is, accordingly, van object of the invention to provide an improved control circuit which is capable of controlling the current through an electric circuit in re- United States Patent 3,263,158 Patented July 26, 1966 ice sponse to variations in the impedance of a sensing transducer element coupled thereto.

A more particular object of the present invention is to provide an improved thermal protective and/ or control circuit for electric motors and the like, which responds to resistance increases of a thermal responsive element, such as a positive temperature coeiicient thermistor, to provide a desired control effect.

Another object of the invention is to provide such an improved control circuit which utilizes solid state elements and thereby obviates the necessity for mechanically moving parts or contacts.

Another object of the invention is to provide such an improved control circuit which is capable of controlling relatively heavy loads in response to relatively low level signals.

Another object of the invention is to provide such an improved solid state con-trol circuit in which the solid state element thereof is completely isolated from the line voltage; so that a relatively inexpensive, low voltage solid state element can be used in conjunction with relatively Ahigh line voltages.

Yet another object of the invent-ion is to provide such an improved solid state control circuit in which the sensing and control circuits and all the Isolid state active units may be operated at relatively low voltages:

A still further object of the invention is to provide such an improved protective and/ or control circuit which exhibits electronic hysteresis in the form of a desirable lag between its drop out and pull in operational modes, so as to avoid chatter of the controlled contacter and other undesirable eiects.

Other objects and advantages of the invention will become apparent from a consideration of the following description, when the description is read in conjunction with the accompanying drawings, in which: v

FIGURE 1 is a diagram, partly in block form and partly in circuit lform, illustrating the Imanner in which the protective circuit of the present invention may be utilized to protect a three-phase electric motor;

FIGURE 2 is a fragmentary circuit diagram useful in explaining the control circuit lconcept of the present invention and representing the basic principles on which the concept is predicated; and

FIGURE 3 is a circuit diagram of one form of the vcontrol circuit of the invention.

As mentioned above, although the control circuit of the present invention is illustrated herein as forming a protective means for an electric motor, the control circuit may have a wide variety of uses. The circuit basically is one which responds to an increase in resistance so as to provide a decrease in current tlow from a source to a controlled element. However, it will be evident -as the y description proceeds, that the circuit may be constructed to respond to a decrease in resistance so as toachieve -this purpose. Moreover, the control circuit of the present invention may be used, not only tocontrol a contactor associated with an electric motor, as will be described herein, but to control Ithe motor directly, or to Acontrol any other electrical instrumentality which is to In the illustrated em-' J to the actuating coil M of a contactor 12. The contactor 12 is illustrated as interposed `in a three-phase line, and it includes three normally open contacts M1, M2 and M3.

During normal operation of the system, the coil M of the contactor is energized, and it holds the three contacts M1, M2 and M3 closed. The three-phase line supplies power to an usual three-phase motor 14. The motor 14 is energized, so long as the contacts M1, M2 and M3 are held closed. The motor is shown schematically as including the usual Y-connected field winding 14a, 14]? and 14C.

In the practice of one embodiment of the invention, a pair of positive temperature coefficient thermistors are embedded in two of the field windings 14h and 14C of the motor 14, and these thermistors are designated 16 and 1S. The thermistors 16 and 18 are connected in series, and the series string is connected to control ter- `minals X1 and X2 of the motor protective circuit 10.

It is to be understood, of course, that the thermistors 16 and 18 could be placed at other portions of the motor 14, and that more or less of these thermistors could be used. The -punpose of the thermistors is to respond to any abnormal increase in temperature in the motor, so as to exhibit a substantial increase in resistance and thereby to provide a control effect for the motor protective circuit 10. This control effect, as will be described, causes the circuit to de-energize the winding M of the contacter and open the contacts M1, M2 and M3.

As mentioned above, the fragmentary circuit of FIG- URE 2 represents the basic principles on which the circuit 10 of the present invention is based. As will be described in conjunction with FIGURE` 3, the motor protective or control circuit 10 includes a solid state switching -device such as a silicon control-led rectifier 20. The silicon controlled rectifier has the usual anode, cathode and gate electrodes. In the practice of the system 4of the present invention, the silicon controlled rectifier 20 Vis normally conductive. Then, when an abnormal condition arises, such as an increase of temperature, the silicon controlled rectifier is rendered non-conductive.

The control circuit of the invention may be considered as including a pair of unidirectional potential sources 22 and 24 (FIGURE 2). The negative terminal of the source 22 is connected to the positive termina-l of the source 24. It will become evident from the description of the circuit of FIGURE 3, that these unidirectional sources 22 and 24 actually produce unfiltered pulsating direct-current potentials derived from rectifying diodes. The negative terminal of the source 24 is connected to the gate electrode of the silicon controlled rectifier 20 through a balancing resistor R1. The positive terminal of the source 24, and negative terminal of the source 422, are connected to the cathode of the device 20. A

22 is applied to the gate electrode of the silicon controlled rectifier 20. If the value of the balancing resistor R1 is .made somewhat higher than the thermistor resistance R11, the net bias on the gate electrode will be positive, so that the silicon controlled rectifier will be conductive.

However, in the event that the resistance of the thermister 26 increases materially, due to an abnormal temperature rise in the protected equipment, the bias on the y gate electrode of the silicon controlled rectifier 20 will swing negative, so that the silicon controlled rectifier will be cut-off.

A control circuit constructed in accordance with one embodiment of the invention is shown in FIGURE 3. An important feature of the circuit of FIGURE 3 is that the silicon controlled rectifier 20 illustrated therein is completely isolated from the line. This means that inexpensive silicon controlled rectifiers, or equivalent switching devices, having relatively low voltage characteristics may be used in conjunction with a relatively high line voltage.

Another feature of the circuit of FIGURE 3 is the fact that it exhibits electronic hysteresis That is, the circuit requires a lower resistance to achieve its pull-in operational mode, than required for its drop-out mode. This permits a lag between the on and o operation of the system which, as mentioned above, obviates chatter of the contactor contacts, and other undersirable effects.

As described above, the motor protective circuit 10 of FIGURE 3 includes a pair of input terminals L1 and L2, and it includes a pair of output terminals T1 and T2. The input terminals L1 and L2 are connected to an appropriate voltage source, such as the usual 11S-volt alterhating-current line. The output terminals T1 and T2 may be connected, for example, t-o the coil M of the contacter 12 in FIGURE l.

The protective or control circuit of FIGURE 3 includes a first transformer 50 which has a primary winding 50a connected to the input terminals L1 and L2 so as to be energized by the power from the alternatingcurrent source. The circuit also includes a second transformer 52 which has a prim-ary winding 52a connected in the series in the connection between the input terminals L1, L2 and the output terminals T1 and T2.

As will be described, under normal operating conditions, the primary winding 52a of thetransformer 52 exhibits a relatively low impedance. This means that sufficient current is supplied to the coil M of the contacter 12 in FIGURE 1 to maintain the contactor contacts M1, M2 and M3 in a closed condition. However, upon the occurrence of an abnormal temperature rise in the protected equipment, the impedance of the primary winding 52a of the transformer 52 rises due to the switching off of the controlled rectifier 20, as will be described, so that insufficient current flows in the coil M to maintain the circuit breaker contacts closed and the circuit enters its drop-out operational mode. This latter mode continues until the abnornal temperature rise disappears.

Due to the above mentioned electronic hysteresis eff-ect, a lag is provided (in a manner to be described) between the drop-out and pull-in operational modes of the circuit. This means that the abnormal temperature rise which created the initial condition must be reduced appreciably before the circuit willagain enter its pull-in mode, so as to energize again the contactor coil M.

The transformer 5f) has a secondary winding Sb. A pair of diode rectifiers 54 and 56 have their anodes connected to the respective extremities of the secondary winding of the transformer 50. The cathodes of the diodes 54 and 56 are both connected to the control terminal X1. The other control terminal X2 is connected to the balancing resistor R1 and to the gate electrode of the silicon controlled rectifier 20.

The transformer 52 has a first secondary winding 52b and a second secondary winding 52C. The balancing resistor R1 is connected to an intermediate tap on the secondary winding 52h of the transformer S2 A pair of diodes 58 4and 6l) have their anodes connected to the extremities of the secondary winding 52b, and their cathodes connected respectively to the extremities of the secondary winding 50h.

A11 intermediate tap on the secondary winding 50b is connected to the anodes of a pair of `diode rectiers 62 iand 64 and to the cathode of the silicon controlled rectifier 2f). This latter connection may fbe grounded, if so desired, .as shown. However, the control system may be floating if so desired. The cathode of the diode re'ctifiers 62 and 64 are connected respectively to the extremities ofthe secondaryrwin-ding 52C, and lthe anode of the silicon controlled rectifier 20 is connected to an intermediate tap on the winding.

As |will be described, the transformer 5-2y functions as a variable impedance. A transformer with low losses `and close coupling between the primary |winding 52a and the secondary rwinding 52e is suitable lfor the purpose.

Under normal conditions, the silicon controlled rectifier 20 yis conductive, as mentioned above, so as to shortcircuit alternate halves of the secondary winding 52e to the alternating current 'voltage .induced yacross the secondary by the current through the primary winding 52a. This means that the impedance of the primary winding 52a is relatively lo-W, so that sufficient current is supplied -to the coil M of the contact-or 12 in FIG-URE. 1 to cause the contacts to close.

However, when an abnormal condition occurs, the silicon controlled rectifier `20 is rendered non-conductive, and the impedance in the primary lwinding l52a rises appreciably. For this latter condition, there is insufficient current flow from the input terminals L1 rand L2 of the circuit to the output terminals T1 and T2 through the coil M so as to hold the contactor contacts closed. Therefore, the contactor trips.

The control effect rwhich causes the silicon controlled rectifier to become nonaconductive is rendered by the thermistors 16 and 18, for example, which are connected as a series string across the terminals X1 and X2, the resistance of which is designated RX. ASo long as the resistance RX of these thermistors -is relatively low, compared 'with the resistance of R1, a net positive bias is applied from the secondary winding 50b and through a half bridge network, composed of the diode rectifiers 54 and |56 and the resistance RX exhibited by thermistors, to the gate electrode of the silicon controlled rectifier so as to maintain the silicon controlled recti fier conductive.

The positive voltage introduced by the Ibridge network circuit elements mentioned in the previous paragraph, is greater than the negative voltage produced by the other half of the rbridge network from the secondary Winding 50h; the latter half of the fbridge network lbeing composed of the diode `rectifiers 58 and y60, and the balancing resistor R1. The balancing resistor R1 is adjusted so that its value is greater than RX at normal Winding temperatures to assure that the net bias on the gate electrode of the silicon controlled rectifier 20 is positive during normal operating conditions. t

However, 'when an increase in temperature, or other monitored effects, causes the resistance RX to increase above R1, the laforementioned bridge network is unbalanced in the opposite direction. The net bias on the gate electrode of the silicon controlled rectifier 20 is then driven negative, `due to the negative potential applied through the port-ion of the bridge network including the :balancing resistor `R1. Therefore, under the latter condition, the silicon controlled rectifier 20 is rendered non-conductive.

The secondary Winding 52h of the transformer 52 nrovides the aforementioned electronic hysteresis effect. During normal operation7 the 'winding 52e of the transformer 52 is short-circuited by the conduction of the silicon controlled-rectifier 20 so that relatively little voltage is induced in 'winding 52h. Thus the volt-age across the balancing resistor R1 is essentially the direct-current voltage derived through the diode rectifiers 58 and 60 from the sec-ondary 'win-ding i50b of the other transformer 50. When the silicon controlled-rectifier is rendered non-conductive by fan increase -in RX, the reactance of winding 52a becomes much higher than the load reactance of the -contactor coil M. Therefore, most of the line voltage .across terminals L1 and L1 appears across winding "52a, inducing an appreciable voltage across winding =52b. Winding l5211 is phased in such a manner as to 6 increase the 'negative voltage applied to R1 and thus to the. gate of the silicon controlled rectifier 20. In thisf manner the resistance of the controlling elements Rx must drop to a value lower than the point at which nonconduction of controlled rectifier 20 occurred before the netv voltage at the Igate terminal of controlled rectifier 20 again ybecomes positive and conduction of the con-r trolled rectifier resumes. In this manner the desired lag between circuit drop-out and pull-in, or electronic hysteresis effect, is provided.

As mentioned above, although the control circuit of the invention is illustrated as a protective circuit 10 used in conjunction with a contactor system, it is obvious that the control circuit has -a 'wide utility for control and protective applications Als-o, although the control circuit is illustrated as responding to a'positive resistancechange RX, it is obvious that the elements R1 and RX can be interchanged, so'that the control circuit r'will respond to a negative resistance increment. lIn other words, negative temperature' coefiicient lthermistors can be used in the modified circuit.

It is also evident that although a particular embodi ment of the invention has been shown and described, modifications may 'be made'. The following claims are intended to cover all modifications which .fall within the scope of the invention.

What is claimed is:

1. An electric control circuit including in combination: input 'terminal means for connecting the control circuit to a source of alternating-current voltage; output terminal means, a first transformer having a primary 'winding connected to said input terminal means and having a secondary winding; a second transformer having a primary -winding and having a secondary Winding; circuit means including the primary of said second'transformer connecting said input terminal means to said output terminal means; switching means connected to said secondary iwinding of said second transformer for selectively reducing the impedance of the primary winding of said second transformer so as to Iincrease the current :dow therethrough; a pair of control terminals for connecting the control circuit across a vari-able impedance sensing element; means vfor connecting the secondary Winding vof said first transformer to one of said control terminals; and means for connecting the other of said control terminals to said switching means to actuate said switchingA means -in response to a variation in the impedance of said sensingelement.

2. The control circuit `defined in clai-m 1 `in which said switching means comprises a silicon controlled rectifier.

3. An electric control circuit including in combination: input terminal means ffor connecting th'e control circuit to a source of alternating-current voltage; output terminal means; la first transformer having a primary winding connected to said input terminal means and having a secondary winding; a second transformer having a primary winding and a second-ary lwinding; circuit means connecting said input terminal means to said output terminal means and including said primary winding of said second transformer; first diode rectifier means connected to said secondary winding of said second transvformer and to an intermediate tap on said secondary winding of said first transformer; a silicon controlled rectifier switching Idevice connected to an intermediate tap of said secondary Winding of said second transformer and to said first diode rectifier means `for selectively reducing the impe-dance of the primary 'winding of said second transformer to increase the current fiow therethrough; a pair of control terminals for connecting the control circuit across a variable resistance sensing element; a second diode rectifier means for connecting the secondary winding of said first transformer to one of said control terminals; and means Ifor connecting the other of said control terminals to said silicon controlled rectifier switching device to actuate said device in response to a resistance variation in said sensing element.

4. An electric control circuit including in combination: input terminal means :for connecting the control circuit to a source of alternating-current voltage; output terminal means; a first transformer having a primary winding connected to said input terminal means and having a secondary winding; a second transformer having a priunary winding and la secondary winding; circuit means @including the primary winding of said second trans- Iformer connecting said input terminal means to said output terminal means; switching means connected to said secondary winding of said second transformer for selectively reducing the impedance of the primary winding of said sec-ond transformer to increase the current flow therethrough; a pair of control terminals yfor connecting the control circuit across a variable resistance sensing element; and bridge network means including first diode rectifier means -for connecting the secondary winding of .said first transformer to one of said control terminals,

second diode rectifier means and balancing resistance means `for connecting the secondary winding of said first transformer to said switching means to render said switching means normally conductive, and ifurther means connecting the other of said control terminals to said switching means to render said switching means non-conductive in response to a resistance variation in said sensing element.

5. The control circuit defined in claim 4 in which said second transformer includes a second secondary winding connected in circuit 'with said balancing resistance means for providing an additional voltage thereacross upon the rendering of the aforesaid switching means nonconductive.

6. An electric control circuit including in combination: input terminal means for connecting the control circuit to a source of alternating-current voltage; output terminal means; a first transformer having `a primary winding connected across said input terminal means and having a secondary winding; `a second transformer having a primary winding and first and second secondary windings; circuit means including said primary winding of said second transformer connecting said input terminal means to said output terminal means; means including a silicon controlled rectifier switching device connected to said first secondary winding of said second transformer for selectively reducing the impedance of said primary winding of said second transformer to increase the current fiow from said input terminal means to said output terminal means; ya pair of control terminals for connection to a variable resistance sensing element; and a bridge network including first diode rectifier means connecting said secondary winding of said first transformer to one of said control terminals, second diode rectifier means and balancing resistance means connecting said ,secondary 'winding of said lfirst transformer through said second secondary winding of said second transformer to said silicon controlled rectifier device to cause said device to be normally conductive, and further means connecting the other of said control terminals to said silicon controlled rectifier device to cause said device to become non-conductive in response t-o a resistance variation in said sensing element.

7. An electric control circuit including in combination: input terminal means for connecting the control circuit to a source of alternatin-g-current voltage; output terminal means; `a rrst transformer having a primary Winding connected across said input terminal means and having a secondary winding; a second transformer having a primary winding and first and second secondary windings; circuit means including said primary winding of said secon'd transformer connecting said input terminal means to said output terminal means; means including a silicon controlled rectifier switching device having a gate electrode, having an anode `connected to an intermediate tap on said first secondary winding of said second transformer, vand having a cathode connected to van intermediate t-ap on said secondary Winding of said first transformer for selectively reducing the impedance of said primary winding of said second transformer and increase the current iiiowfrom said input terminal means to said output terminal means; a pair of control terminals for connection to a variable resistance sensing element; and a .bridge network including first di-ode rectifier means connecting the extremities of said secondary winding of said first transformer to one of said control terminals, second diode rectifier means Vand .balancing resistance means connecting the extremities of said sec-` ondary winding of said first transformer through said second secondary winding of said second transformer to the gate electrode of said silicon controlled rectifier device to cause said `device to vbe normally conductive, and means connecting the other of said control terminals to the gate electrode of said silicon controlled rectifier device to cause said `device to become non-conductive in response to a resistance variation in said sensing element.

References Cited by the Examiner UNITED STATES PATENTS 2,884,578 4/1959 Bradburn et al. 318-513 X 2,975,976 3/1961 Smith et Ial. 32'3--89 X 3,071,718 1/1963 Gordon 318-473 3,182,249 5/1965 Pahlavan 323-87 LLOYD MCCOLLUM, Primary Examiner.

W. E. RAY, Assistant Examiner. 

1. AN ELECTRIC CONTROL CIRCUIT INCLUDING IN COMBINATION: INPUT TERMINAL MEANS FOR CONNECTING THE CONTROL CIRCUIT TO A SOURCE OF ALTERNATING-CURRENT VOLTAGE; OUTPUT TERMINAL MEANS, A FIRST TRANSFORMER HAVING A PRIMARY WINDING CONNECTED TO SAID INPUT TERMINAL MEANS AND HAVING A SECONDARY WINDING; A SECOND TRANSFORMER HAVING A PRIMARY WINDING AND HAVING A SECONDARY WINDING; CIRCUIT MEANS INCLUDING THE PRIMARY OF SAID SECOND TRANSFORMER CONNECTING SAID INPUT TERMINAL MEANS TO SAID OUTPUT TERMINAL MEANS; SWITCHING MEANS CONNECTED TO SAID SECONDARY WINDING OF SAID SECOND TRANSFORMER FOR SELECTIVELY REDUCING THE IMPEDANCE OF THE PRIMARY WINDING OF SAID SECOND TRANSFORMER SO AS TO INCREASE THE CURRENT FLOW THERETHROUGH; A PAIR OF CONTROL TERMINALS FOR CONNECTING THE CONTROL CIRCUIT ACROSS A VARIABLE IMPEDANCE SENSING ELEMENT; MEANS FOR CONNECTING THE SECONDARY WINDING OF SAID FIRST TRANSFORMER TO ONE OF SAID CONTROL TERMINALS; AND MEANS FOR CONNECTING THE OTHER OF SAID CONTROL TERMINALS OF SAID SWITCHING MEANS TO ACTUATE SAID SWITCHING MEANS IN RESPONSE TO A VARIATION IN THE IMPEDANCE OF SAID SENSING ELEMENT. 